Source/drain feature to contact interfaces

ABSTRACT

Examples of an integrated circuit with an interface between a source/drain feature and a contact and examples of a method for forming the integrated circuit are provided herein. In some examples, a substrate is received having a source/drain feature disposed on the substrate. The source/drain feature includes a first semiconductor element and a second semiconductor element. The first semiconductor element of the source/drain feature is oxidized to produce an oxide of the first semiconductor element on the source/drain feature and a region of the source/drain feature with a greater concentration of the second semiconductor element than a remainder of the source/drain feature. The oxide of the first semiconductor element is removed, and a contact is formed that is electrically coupled to the source/drain feature. In some such embodiments, the first semiconductor element includes silicon and the second semiconductor element includes germanium.

PRIORITY DATA

The present application claims the benefit of U.S. ProvisionalApplication No. 62/751,038, entitled “Source/Drain Feature to ContactInterfaces,” filed Oct. 26, 2018, herein incorporated by reference inits entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. In the course of IC evolution, functional density (i.e., thenumber of interconnected devices per chip area) has generally increasedwhile geometry size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling downprocess generally provides benefits by increasing production efficiencyand lowering associated costs. However, each iteration of size reductionpresents greater challenges to both design and manufacturing. Throughadvances in these areas, increasingly complex designs are beingfabricated with precision and reliability.

As technology advances, parasitic effects, such as undesirableresistances and capacitances, have become more prominent. Theseparasitic effects may have a greater impact with each generation ofimprovements because the new techniques form smaller devices that arecloser together and operate at lower voltages. As an example,undesirable resistance may occur at an interface between conductivefeatures or at an interface between a conductive feature and a circuitfeature such as a gate, source, or drain. The resistance of such aninterface may be due to the quality of the interface as well as thecomposition of the materials at the interface, and the resistance mayincrease as the size of the interface decreases.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a flow diagram of a method of fabricating a workpiece with asource/drain interface according to various aspects of the presentdisclosure.

FIGS. 2-8 are cross-sectional diagrams of a workpiece, taken along afin-length direction, at points in a method of fabrication according tovarious aspects of the present disclosure.

FIGS. 9A-9B are flow diagrams of a method of fabricating a workpiecewith a source/drain interface according to various aspects of thepresent disclosure.

FIGS. 10-22 are cross-sectional diagrams of a workpiece, taken along afin-length direction, at points in a method of fabrication according tovarious aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact.Moreover, the formation of a feature connected to and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact.

In addition, spatially relative terms, for example, “lower,” “upper,”“horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,”“down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of thepresent disclosure of one features relationship to another feature. Thespatially relative terms are intended to cover different orientations ofthe device including the features. In addition, the present disclosuremay repeat reference numerals and/or letters in the various examples.This repetition is for the purpose of simplicity and clarity and doesnot in itself dictate a relationship between the various embodimentsand/or configurations beyond the extent noted.

Advances in integrated circuit design and fabrication have deliveredimprovements in circuit speed and efficiency. However, despite newstructures and new fabrication techniques, transistors and other circuitelements still experience significant losses and inefficiencies. Infact, some parasitic effects increase when devices shrink due to smallerconductors with higher resistance, thinner dielectrics with reducedinsulation, and/or other factors. These parasitic effects may wasteenergy, produce excess heat, reduce maximum operating frequency, and/orincrease minimum operating voltage. In extreme cases, they may lead topremature device failure.

One particular source of parasitic resistance is the interface between asource/drain feature of a circuit device, such as a Field EffectTransistor (FET) and/or a Fin-like FET (FinFET), and a contact thatelectrically couples to the source/drain feature. The resistance at theinterface may be particularly high for source/drain features thatcontain silicon-germanium alloy semiconductors. The present disclosureprovides a technique for forming an integrated circuit device thatincludes performing an oxidation process on the source/drain features tocreate a germanium-rich layer at the top of the source/drain features.This germanium-rich layer is near where a contact will couple, and hasbeen determined to lower the resistance at the interface with thecontact.

Furthermore, in some examples, the technique selectively forms thegermanium-rich layer in SiGe-containing source/drain features of somedevices without adversely impacting Si-containing source/drain featuresof other devices on the same workpiece. This may avoid a separatemasking step to protect the other devices. Moreover, in some examples,the technique selectively forms the germanium-rich layer withoutadditional epitaxial deposition and/or implantation processes. Epitaxyand implantation may add to the fabrication cost and contribute to thethermal budget, and both types of process may cause damage to thesurrounding structures. Accordingly, it is beneficial to avoidadditional epitaxial deposition and implantation processes wherepossible. These benefits, however, are merely examples, and unlessotherwise noted, no embodiment is required to provide any particularadvantage.

Examples of an integrated circuit with germanide source/drain interfacesand an example of a method of forming such a circuit are described withreference to FIGS. 1-8. In that regard, FIG. 1 is a flow diagram of amethod 100 of fabricating a workpiece with a source/drain interfaceaccording to various aspects of the present disclosure. Additional stepscan be provided before, during, and after the method 100, and some ofthe steps described can be replaced or eliminated for other embodimentsof the method 100.

FIGS. 2-8 are cross-sectional diagrams of the workpiece 200, taken alonga fin-length direction, at points in the method 100 of fabricationaccording to various aspects of the present disclosure. FIGS. 2-8 havebeen simplified for the sake of clarity and to better illustrate theconcepts of the present disclosure. Additional features may beincorporated into the workpiece 200, and some of the features describedbelow may be replaced or eliminated for other embodiments of theworkpiece 200.

Referring to block 102 of FIG. 1 and to FIG. 2, the workpiece 200 isreceived. The workpiece 200 includes a substrate 202 upon which devicesare to be formed. In various examples, the substrate 202 includes anelementary (single element) semiconductor, such as silicon or germaniumin a crystalline structure; a compound semiconductor, such as siliconcarbide, gallium arsenic, gallium phosphide, indium phosphide, indiumarsenide, and/or indium antimonide; an alloy semiconductor such as SiGe,GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; anon-semiconductor material, such as soda-lime glass, fused silica, fusedquartz, and/or calcium fluoride (CaF₂); and/or combinations thereof.

The substrate 202 may be uniform in composition or may include variouslayers, some of which may be selectively etched to form the fins. Thelayers may have similar or different compositions, and in variousembodiments, some substrate layers have non-uniform compositions toinduce device strain and thereby tune device performance. Examples oflayered substrates also include silicon-on-insulator (SOI) substrates202. In some such examples, an insulator layer of an SOI substrate 202includes a semiconductor oxide, a semiconductor nitride, a semiconductoroxynitride, a semiconductor carbide, and/or other suitable insulatormaterials.

Doped regions, such as wells, may be formed on the substrate 202, andsome regions of the substrate 202 may be doped with p-type dopants, suchas boron, BF₂, or indium while other regions of the substrate 202 may bedoped with n-type dopants, such as phosphorus or arsenic; and/or othersuitable dopants including combinations thereof. The doping of aparticular region of the substrate 202 may depend on the devices to beformed on the region. In an example, the substrate 202 includes a firstregion 204 for forming n-channel (nFET) devices and a second region 206for forming p-channel (pFET) devices.

In some examples, the devices to be formed on the substrate 202 extendout of the substrate 202. For example, Fin-like Field Effect Transistors(FinFETs) and/or other non-planar devices may be formed on device fins208 disposed on the substrate 202. The device fins 208 arerepresentative of any raised feature and include FinFET device fins 208as well as fins 208 for forming other raised active and passive devicesupon the substrate 202. The fins 208 may be formed by etching portionsof the substrate 202, by depositing various layers on the substrate 202and etching the layers, and/or by other suitable techniques. Forexample, the fins 208 may be patterned using one or morephotolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the fins.

The fins 208 may be similar in composition to the substrate 202 or maybe different therefrom. For example, in some embodiments, the substrate202 includes primarily silicon, while the fins 208 include one or morelayers that are primarily germanium or a SiGe semiconductor. In someembodiments, the substrate 202 includes a SiGe semiconductor, and thefins 208 include one or more layers that include a SiGe semiconductorwith a different ratio of silicon to germanium.

Each device fin 208 may include any number of circuit devices, such asFinFETs, that, in turn, each include a pair of opposing source/drainfeatures (e.g., nFET source/drain features 210 and pFET source/drainfeatures 212) formed on the fin 208 and separated by a channel region214. The source/drain features 210 and 212 of the FinFETs may include anepitaxially-grown semiconductor and one or more dopants. Both thesemiconductor and the dopants may differ between the nFET source/drainfeatures 210 of the nFET region 204 and the pFET source/drain features212 of the pFET region 206. In some examples, the nFET source/drainfeatures 210 include an elementary semiconductor (e.g., silicon) andn-type dopants (e.g., phosphorus and/or arsenic). In contrast, theexample pFET source/drain features 212 include an alloy semiconductor(e.g., SiGe) and p-type dopants (e.g., boron, BF₂, and/or indium).Accordingly, in various such examples, the nFET source/drain features210 include SiP, SiCP, and/or SiAs, and the pFET source/drain features212 include SiGeB, and/or SiGeIn, with a relatively high concentrationof Ge relative to Si (e.g., excluding dopants, a concentration of Gegreater than about 50 atomic percent).

The flow of carriers (electrons for an n-channel FinFET and holes for ap-channel FinFET) from the source to the drain is controlled by avoltage applied to a gate stack 216 adjacent to and overwrapping thechannel region 214. The raised channel region 214 provides a largersurface area proximate to the gate stack 216 than comparable planardevices. This strengthens the electromagnetic field interactions betweenthe gate stack 216 and the channel region 214, which may reduce leakageand short channel effects associated with smaller devices. Thus in manyembodiments, FinFETs and other nonplanar devices deliver betterperformance in a smaller footprint than their planar counterparts do.

An example gate stack 216 may include an interfacial layer 218 disposedon the top and side surfaces of the channel regions 214. The interfaciallayer 218 may include an interfacial material, such as a semiconductoroxide, semiconductor nitride, semiconductor oxynitride, othersemiconductor dielectrics, other suitable interfacial materials, and/orcombinations thereof. The gate stack 216 may include a gate dielectric220 disposed on the interfacial layer 218. The gate dielectric 220 mayalso extend vertically along the sides of the gate stack 216. The gatedielectric 220 may include one or more dielectric materials, which arecommonly characterized by their dielectric constant relative to silicondioxide. In some embodiments, the gate dielectric 220 includes a high-kdielectric material, such as HfO₂, HfSiO, HfSiON, HfTaO, HfIiO, HfZrO,zirconium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃)alloy, other suitable high-k dielectric materials, and/or combinationsthereof. Additionally or in the alternative, the gate dielectric 220 mayinclude other dielectrics, such as a semiconductor oxide, semiconductornitride, semiconductor oxynitride, semiconductor carbide, amorphouscarbon, TEOS, other suitable dielectric material, and/or combinationsthereof. The gate dielectric 220 may be formed to any suitablethickness, and in some examples, the gate dielectric 220 has a thicknessof between about 0.1 nm and about 3 nm.

A gate electrode is disposed on the gate dielectric 220. The gateelectrode may include a number of different conductive layers, of whichthree exemplary layers (a capping layer 222, work function layer(s) 224,and an electrode fill 226) are shown. With respect to the capping layer222, it may include any suitable conductive material including metals(e.g., W, Al, Ta, Ti, Ni, Cu, Co, etc.), metal nitrides, and/or metalsilicon nitrides. In various embodiments, the capping layer 222 includesTaSiN, TaN, and/or TiN.

The gate electrode may include one or more work function layers 224 onthe capping layer 222. Suitable work function layer 224 materialsinclude n-type and/or p-type work function materials based on the typeof device. Exemplary p-type work function metals include TiN, TaN, Ru,Mo, Al, WN, ZrSi₂, MoSi₂, TaSi₂, NiSi₂, WN, other suitable p-type workfunction materials, and/or combinations thereof. Exemplary n-type workfunction metals include Ti, Ag, TaAl, TaAlC, TiAlN, TaC, TaCN, TaSiN,Mn, Zr, other suitable n-type work function materials, and/orcombinations thereof.

The gate electrode may also include an electrode fill 226 on the workfunction layer(s) 224. The electrode fill 226 may include any suitablematerial including metals (e.g., W, Al, Ta, Ti, Ni, Cu, Co, etc.), metaloxides, metal nitrides, and/or combinations thereof, and in an example,the electrode fill 226 includes tungsten.

In some examples, the gate stack 216 includes a gate cap 228 on top ofthe gate dielectric 220, the capping layer 222, the work functionlayer(s) 224, and/or the electrode fill 226. The gate cap 228 mayinclude any suitable material, such as: a dielectric material (e.g., asemiconductor oxide, a semiconductor nitride, a semiconductoroxynitride, a semiconductor carbide, a semiconductor oxycarbonitride,etc.), polysilicon, SOG, TEOS, PE-oxide, HARP-formed oxide, and/or othersuitable material. In some examples, the gate cap 228 includes siliconoxycarbonitride. In some examples, the gate cap 228 has a thicknessbetween about 1 nm and about 10 nm.

Sidewall spacers 230 are disposed on the side surfaces of the gatestacks 216. The sidewall spacers 230 may be used to offset thesource/drain features 210 and 212 and to control the source/drainjunction profile. In various examples, the sidewall spacers 230 includeone or more layers of suitable materials, such as a dielectric material(e.g., a semiconductor oxide, a semiconductor nitride, a semiconductoroxynitride, a semiconductor carbide, a semiconductor oxycarbonitride,etc.), Spin On Glass (SOG), tetraethylorthosilicate (TEOS), PlasmaEnhanced CVD oxide (PE-oxide), High-Aspect-Ratio-Process (HARP)-formedoxide, and/or other suitable material. In one such embodiment, thesidewall spacers 230 each include a first layer of silicon oxide, asecond layer of silicon nitride disposed on the first layer, and a thirdlayer of silicon oxide disposed on the second layer. In the embodiment,each layer of the sidewall spacers 230 has a thickness between about 1nm and about 10 nm.

The workpiece 200 may also include a contact-etch stop layer (CESL) 232disposed on the source/drain features 210 and 212 and alongside thesidewall spacers 230. The CESL 232 may include a dielectric (e.g., asemiconductor oxide, a semiconductor nitride, a semiconductoroxynitride, a semiconductor carbide, etc.) and/or other suitablematerial, and in various embodiments, the CESL 232 includes SiN, SiO,SiON, and/or SiC. In some examples, the CESL 232 has a thickness betweenabout 1 nm and about 50 nm.

One or more Inter-Level Dielectric (ILD) layers (e.g., layers 234 and236) are disposed on the source/drain features 210 and 212 and gatestacks 216 of the workpiece 200. The ILD layers 234 and 236 act asinsulators that support and isolate conductive traces of an electricalmulti-level interconnect structure. In turn, the multi-levelinterconnect structure electrically interconnects elements of theworkpiece 200, such as the source/drain features 210 and 212 and thegate stacks 216. The ILD layers 234 and 236 may include a dielectricmaterial (e.g., a semiconductor oxide, a semiconductor nitride, asemiconductor oxynitride, a semiconductor carbide, etc.), SOG,fluoride-doped silicate glass (FSG), phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), Black Diamond®, Xerogel, Aerogel,amorphous fluorinated carbon, parylene, BCB, SiLK®, and/or combinationsthereof.

Referring to block 104 of FIG. 1 and to FIG. 3, trenches 302 are formedin the ILD layers 234 and 236 for source/drain contacts. The trenches302 expose the source/drain features 210 and 212 at locations whereconductive features of the interconnect are to be formed. In some suchexamples, this includes forming a photoresist 304 on the workpiece 200and patterning the photoresist 304 in a photolithographic process toselectively expose portions of the ILD layers 234 and 236 to etch.

In one embodiment, a photolithographic system exposes the photoresist304 to radiation in a particular pattern determined by a mask. Lightpassing through or reflecting off the mask strikes the photoresist 304thereby transferring a pattern formed on the mask to the photoresist304. In other such embodiments, the photoresist 304 is exposed using adirect write or maskless lithographic technique, such as laserpatterning, e-beam patterning, and/or ion-beam patterning. Once exposed,the photoresist 304 is developed leaving the exposed portions of theresist, or in alternative examples, leaving the unexposed portions ofthe resist. An exemplary patterning process includes soft baking of thephotoresist 304, mask aligning, exposure, post-exposure baking,developing the photoresist 304, rinsing, and drying (e.g., hard baking).

The portions of the ILD layers 234 and 236 exposed by the photoresist304 are then etched using any suitable etching technique such as wetetching, dry etching, RIE, ashing, and/or other etching methods. In someembodiments, the etching process includes dry etching using anoxygen-based etchant, a fluorine-based etchant (e.g., CF₄, SF₆, CH₂F₂,CHF₃, and/or C₂F₆), a chlorine-based etchant (e.g., Cl₂, CHCl₃, CCl₄,and/or BCl₃), a bromine-based etchant (e.g., HBr and/or CHBR₃), aniodine-based etchant, other suitable etchant gases or plasmas, and/orcombinations thereof.

Any remaining photoresist 304 may be removed after etching the trenches302.

The trenches 302 may remove some or all of the ILD layer 234 and CESL232 above the source/drain features 210 and 212 and may expose the topsurfaces of the source/drain features 210 and 212 in whole or in part.Referring to block 106 of FIG. 1 and to FIG. 4, a controlled oxidationis performed on the workpiece 200 that oxidizes the exposed surfaces ofthe source/drain features 210 and 212. For those source/drain features210 that include a semiconductor that is primarily silicon, theoxidation process may create a layer 402 of SiO_(X) on the surface ofthe source/drain features 210. Because silicon oxidizes more readilythan germanium, an oxidation process may draw silicon out of asilicon-germanium semiconductor to form a layer of predominantly siliconoxide at the surface. In such examples and others, the oxidation processof block 106 creates a layer 402 of SiO_(X) on the surface of the SiGesource/drain features 212 as the process is configured to substantiallyavoid oxidation of the Ge within the source/drain features 212.

As a result of drawing out the silicon from a portion of thesource/drain features 212, the oxidation process may form a Ge-richlayer 404 containing the remaining germanium just under the SiO_(X)layer 402. The Ge-rich layer 404 has a higher concentration of Gerelative to Si than the remainder of the source/drain feature 212. Invarious examples, excluding dopants, the Ge-rich layer 404 containsbetween about 10 atomic percent and about 100 atomic percent Ge. In somesuch examples, the Ge-rich layer 404 contains only Ge and dopants. TheSiO_(X) layer 402 and Ge-rich layer 404 may be formed to have anysuitable thickness, and in various examples, each has a thicknessbetween about 1 nm and about 10 nm. The SiO_(X) layer 402 may besubstantially conformal and follow the contour of the top of thesource/drain features 210 and 212.

The oxidation process may include any suitable oxidizing techniqueincluding dry (O₂) and/or wet (H₂O) oxidation techniques. In someexamples, the oxidation process includes heating the workpiece 200 totemperature between about 250° C. and about 500° C. at a pressurebetween about 0.1 Torr and about 8×10⁵ Torr and introducing an oxygensource such as O₂ or H₂O. The upper oxidation process temperature may belimited by the materials of the gate stack 216. The oxidation processmay be performed for any suitable duration and, in various examples, isperformed for between about 10 minutes and about 24 hours. In this way,the technique reliably forms a monocrystalline Ge-rich layer 404 in thepFET source/drain features 212 of the pFET region 206 without additionalepitaxy or implantation processes.

Referring to block 108 of FIG. 1 and to FIG. 5, the SiO_(x) layers 402are removed in a precleaning process. The precleaning process iscontrolled to avoid significant removal of the Ge-rich layer 404 of thepFET source/drain features 212. The precleaning process may include adry cleaning process, a wet cleaning process, RIE, and/or other suitablecleaning methods. For example, in some embodiments, the precleaningprocess includes a plasma-assisted dry etch process that utilizes NH₃,NF₃, HF, and/or H₂. In some embodiments, the precleaning processincludes a wet cleaning process that utilizes diluted hydrofluoric acid(DHF) solution. To avoid reforming SiO_(x) on the surfaces of thesource/drain features 210 and 212, the precleaning process may beperformed in the same chamber as the subsequent silicidation process.

Referring to block 110 of FIG. 1 and to FIG. 6, a silicide/germanideforming process is performed on the workpiece 200. Thesilicide/germanide process introduces a metal or other conductivematerial into the source/drain features 210 and 212. In particular, theprocess may form a silicide layer 602 in silicon-containing source/drainfeatures 210 in the nFET region 204 and a germanide layer 604 in theGe-rich layer 404 of the source/drain features 212 in the pFET region206.

The silicide/germanide process may include depositing a metal or otherconductor on the workpiece 200. Suitable conductors include Ti, Er, Y,Yb, Eu, Tb, Lu, Th, Sc, Hf, Zr, Tb, Ta, Ni, Co, Pt, W, Ru, and/or othersuitable conductors. The conductor may be deposited by Chemical VaporDeposition (CVD), Atomic Layer Deposition (ALD), Plasma-Enhanced CVD (PECVD), Plasma-Enhanced ALD (PEALD), Physical Vapor Deposition (PVD),and/or other suitable techniques.

In some examples, the silicide/germanide process includes one or morenitridation processes to provide a nitrogen source for a nitridized caplayer. The nitridation creates a barrier against inadvertent oxidationof the underlying materials from ambient oxygen prior to depositingsubsequent materials of the contacts. The nitridation process may beperformed in the same tool and/or the same chamber of the tool used todeposit the conductor. In various examples, a nitrogen-containing gassuch as N₂ and/or NH₃ is supplied at an injection flow rate betweenabout 20 sccm and about 200 sccm at a process temperature between about20° C. and about 120° C. for between about 1 minute and about 30minutes. Where PE CVD is used for nitridation, the plasma power for thePE CVD process may be between about 100 W and about 1000 W. Inert gassessuch as argon or helium may be used for plasma ignition. The flow rateof the nitrogen-containing gas, the relative gas concentrations, theduration, the temperature, the field power, and other process conditionsmay be selected to control the nitrogen concentration of the resultingnitridized cap layer described in more detail below. In variousexamples, the nitridation process is configured to produce a nitridizedcap layer with a nitrogen concentration between about 15 and about 40atomic percent.

The workpiece 200 is annealed to react the conductor with thesource/drain features 210 and 212 to form silicide and/or germanide. Theannealing may also cause one or more nitridized capping layers to formon the silicide and/or germanide. Thereafter, any un-reacted metal maybe removed.

In this manner, the silicide/germanide forming process may form asilicide layer 602 on the silicon-containing source/drain features 210in the nFET region 204. The silicide layer 602 may have any suitablethickness, and in various examples, is between about 1 nm and about 10nm thick. Similarly, the process may form a germanide layer 604 on theSiGE-containing source/drain features 212 in the pFET region 206 and anitridized germanide cap 606 on the germanide layer 604. In variousexamples, the nitridized germanide cap 606 has a nitrogen concentrationbetween about 15 and about 40 atomic percent.

In particular, the process may form the germanide layer 604 and thenitridized germanide cap 606 by consuming the Ge-rich layer 404. In someexamples, only about 2 nm or less of the Ge-rich layer 404 remains. Insome examples, the Ge-rich layer 404 is completely removed. Theresulting germanide layer 604 and nitridized germanide cap 606 may haveany suitable thickness. In various examples, the germanide layer 604 hasa thickness between about 2 nm and about 5 nm, and the nitridizedgermanide cap 606 has a thickness between about 1 nm and about 3 nm. Thegreater concentration of germanium in the Ge-rich layer 404, thegermanide layer 604, and/or the nitridized germanide cap 606 produced bythe present technique has been determined to produce a better qualityinterface with subsequently formed contacts and reduced contactresistance.

Referring to block 112 of FIG. 1 and to FIG. 7, source/drain contacts702 are formed in the trenches 302 that couple to the source/drainfeatures 210 and 212. In particular, the contacts 702 may physically andelectrically couple to the silicide layer 602 of the nFET source/drainfeatures 210 and to the germanide layer 604 and/or nitridized germanidecap 606 of the pFET source/drain features 212. The contacts 702 mayinclude one or more layers of conductive materials such as metals (e.g.,W, Al, Ta, Ti, Ni, Cu, etc.), metal oxides, metal nitrides, and/orcombinations thereof. In some examples, a contact 702 contains a barrierlayer that includes W, Ti, TiN, Ru, and/or combinations thereof andcontains a Cu-containing fill material disposed on the barrier layer. Insome examples, a contact 702 includes tungsten, which is deposited withor without a barrier layer. In some examples, a contact includes acobalt contact material. The material(s) of the contacts 702 may bedeposited by any suitable technique including PVD (e.g., sputtering),CVD, PE CVD, ALD, PEALD, and/or combinations thereof.

Referring to block 114 of FIG. 1 and to FIG. 8, a planarization processmay be performed to remove portions of the contact material that areabove the ILD layer 236.

Referring to block 116 of FIG. 1, the workpiece 200 is provided forfurther fabrication. In various examples, this includes forming aremainder of an electrical interconnect structure, dicing, packaging,and other fabrication processes.

The above examples perform the oxidation of block 106 after the openingof the contact trenches. In further examples, oxidation is performedearlier, after forming the source/drain features. Some such examples aredescribed with reference to FIGS. 9A-22. FIGS. 9A-9B are flow diagramsof a method 900 of fabricating a workpiece with a source/drain interfaceaccording to various aspects of the present disclosure. Additional stepscan be provided before, during, and after the method 900, and some ofthe steps described can be replaced or eliminated for other embodimentsof the method 900.

FIGS. 10-22 are cross-sectional diagrams of the workpiece 1000, takenalong a fin-length direction, at points in the method 900 of fabricationaccording to various aspects of the present disclosure. FIGS. 10-22 havebeen simplified for the sake of clarity and to better illustrate theconcepts of the present disclosure. Additional features may beincorporated into the workpiece 1000, and some of the features describedbelow may be replaced or eliminated for other embodiments of theworkpiece 1000.

Referring to block 902 of FIG. 9A and to FIG. 10, the workpiece 1000 isreceived. At least some portions of the workpiece 1000, such as thesubstrate 202, device fins 208, channel regions 214, and sidewallspacers 230, may be substantially similar to those described above. Theworkpiece 1000 may further include gate stacks 1002 disposed on thechannel regions 214 of the fins 208. In some examples, the gate stacks1002 are functional gate structures. However, when materials of thefunctional gate structure are sensitive to fabrication processes or aredifficult to pattern, a placeholder gate of polysilicon, dielectric,and/or other resilient material may be used during some of thefabrication processes. The placeholder gate is later removed andreplaced with elements of a functional gate (e.g., a gate electrode, agate dielectric layer, an interfacial layer, etc.) in a gate-lastprocess. In such examples, the gate stacks 1002 represent placeholdergates.

To form the source/drain features on opposing sides of the channelregions 214, portions of the fins 208 may be etched and the source/drainfeatures may be epitaxially grown in the resulting recesses. Referringto block 904 of FIG. 9A and to FIG. 11, an etching process is performedon the workpiece 1000 to create source/drain recesses 1102. In someexamples, this includes forming a photoresist on the workpiece 1000 andpatterning the photoresist in a photolithographic process to expose onlythose portions of the workpiece 1000 to be etched.

The etching processes itself may include any suitable etching techniquesuch as wet etching, dry etching, Reactive Ion Etching (RIE), ashing,and/or other etching methods. In some embodiments, the etching processincludes dry etching using an oxygen-based etchant, a fluorine-basedetchant (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-basedetchant (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-based etchant(e.g., HBr and/or CHBR₃), an iodine-based etchant, other suitableetchant gases or plasmas, and/or combinations thereof. In someembodiments, the etching process includes wet etching using dilutedhydrofluoric acid (DHF), potassium hydroxide (KOH) solution, ammonia,hydrofluoric acid (HF), nitric acid (HNO₃), acetic acid (CH₃COOH),and/or other suitable wet etchant(s). In some examples, the etchant isselected to etch the fins 208 without significantly etching surroundingstructures such as the gate stacks 1002 and/or sidewall spacers 230.This may allow the etching to be performed even if the patternedphotoresist is not perfectly aligned.

Any remaining photoresist may be removed after the etching.

Source/drain features may be formed in the nFET region 204 and the pFETregion 206 in any order. In an example, source/drain features are formedin the nFET region 204 first. Referring to block 906 of FIG. 9A and toFIG. 12, a photoresist 1202 is formed on the workpiece 1000 and ispatterned in a photolithographic process to expose the nFET region 204.

Referring to block 908 of FIG. 9A and to FIG. 13, nFET source/drainfeatures 1302 are formed within the source/drain recesses 1102 in thenFET region 204. The nFET source/drain features 1302 may also extend outof the source/drain recesses 1102 to a height above the fins 208. Thesource/drain features 1302 may be substantially similar to the nFETsource/drain features 210 above, and in various examples, the nFETsource/drain features 1302 are formed by a Chemical Vapor Deposition(CVD) deposition technique (e.g., Vapor-Phase Epitaxy (VPE) and/orUltra-High Vacuum CVD (UHV-CVD)), molecular beam epitaxy, and/or othersuitable processes. The epitaxy process may use gaseous and/or liquidprecursors, which interact with a component of the fins 208 (e.g.,silicon or silicon-germanium) to form the nFET source/drain features1302. The semiconductor component of the source/drain features 1302 maybe similar to or different from the remainder of the fin 208. Forexample, Si-containing source/drain features 1302 may be formed on aSiGe-containing fin 208 or vice versa. When the source/drain features1302 and fins 208 contain more than one semiconductor, the ratios may besubstantially similar or different.

The source/drain features 1302 may be in-situ doped to include n-typedopants, such as phosphorus or arsenic, and/or other suitable dopantsincluding combinations thereof. Additionally or in the alternative, thesource/drain features 1302 may be doped using an implantation process(i.e., a junction implant process) after the source/drain features 1302are formed. In various examples, the doped source/drain features 1302include SiP, SiCP, and/or SiAs.

Any remaining photoresist 1202 may be removed after the source/drainfeatures 1302 are formed.

Referring to block 910 of FIG. 9A and to FIG. 14, a photoresist 1402 isformed on the workpiece 1000 and is patterned to expose the pFET region206. Referring to block 912 of FIG. 9A and referring still to FIG. 14,pFET source/drain features 1404 are formed within the source/drainrecesses 1102 in the pFET region 206. The source/drain features 1404 mayalso extend out of the source/drain recesses 1102 to a height above thefins 208. The source/drain features 1404 may be substantially similar tothe pFET source/drain features 212 above, and may be formed by a CVDdeposition technique, molecular beam epitaxy, and/or other suitableprocesses. The semiconductor component of the source/drain features 1404may be similar to or different from the remainder of the fin 208.

The pFET source/drain features 1404 may be in-situ doped to includep-type dopants, such as boron, BF₂, or indium; and/or other suitabledopants including combinations thereof. Additionally or in thealternative, the source/drain features 1404 may be doped using animplantation process (i.e., a junction implant process) after thesource/drain features 1404 are formed. In various examples, the pFETsource/drain features 1404 include SiGeB, and/or SiGeIn, with arelatively high concentration of Ge relative to Si (e.g., excludingdopants, a concentration of Ge greater than about 50 atomic percent).

Any remaining photoresist 1402 may be removed after the source/drainfeatures 1404 are formed.

Referring to block 914 of FIG. 9A and to FIG. 15, an oxidation processis performed on the workpiece 1000. The oxidation process may besubstantially similar to that of block 106 of FIG. 1 and is configuredto introduce oxygen into at least the pFET source/drain features 1404within the pFET region 206. In some examples, the oxidation processfurther introduces oxygen into the nFET source/drain features 1302 ofthe nFET region 204. In some examples, a patterned photoresist layer isused to cover and protect the source/drain features 1302 of the nFETregion 204 to prevent oxidation of the source/drain features 1302.

The oxidation process may include any suitable oxidizing techniqueincluding dry (O₂) and/or wet (H₂O) oxidation techniques. In someexamples, the oxidation process includes heating the workpiece 1000 totemperature between about 250° C. and about 700° C. at a pressurebetween about 0.1 Torr and about 8×10⁵ Torr and introducing an oxygensource such as O₂ or H₂O. The oxidation process may be performed for anysuitable duration and, in various examples, is performed for betweenabout 10 minutes and about 24 hours.

In examples where the primarily silicon nFET source/drain features 1302are oxidized, the oxidation process may create a layer 1502 of SiO_(X)on the surface of the source/drain features 1302. Because siliconoxidizes more readily than germanium, an oxidation process may drawsilicon out of a silicon-germanium semiconductor to form a layer ofpredominantly silicon oxide at the surface. In such examples and others,the oxidation process of block 914 creates the layer 1502 of SiO_(X) onthe surface of the SiGe pFET source/drain features 1404 as the processis configured to substantially avoid oxidation of the Ge within thesource/drain features 1404.

As a result of drawing out the silicon from a portion of the pFETsource/drain features 1404, the oxidation process may form a Ge-richlayer 1504 containing the remaining germanium just under the SiO_(X)layer 1502. The Ge-rich layer 1504 has a higher concentration of Gerelative to Si than the remainder of the source/drain feature 1404. Invarious examples, excluding dopants, the Ge-rich layer 1504 containsbetween about 10 atomic percent and about 100 atomic percent Ge. In somesuch examples, the Ge-rich layer 1504 contains only Ge and dopants.

The SiO_(X) layer 1502 and Ge-rich layer 1504 may be formed to have anysuitable thickness, and in various examples, each has a thicknessbetween about 1 nm and about 10 nm. The SiO_(X) layer 1502 may besubstantially conformal and may follow the contour of the top of thesource/drain features 1302 and 1404. In some such examples, the SiO_(X)layer 1502 over a source/drain feature 1404 has side portions that slopeupwards in a direction away from the sidewall spacers 230 and a centralportion extending between the side portions that is substantiallyhorizontal.

Referring to block 916 of FIG. 9A, a dopant activation process, such asRapid Thermal Annealing (RTA) and/or a laser annealing process, may beperformed on the workpiece 1000 to activate the dopants within thesource/drain features 1302 and 1404.

Referring to block 918 of FIG. 9A and to FIG. 16, a contact-etch stoplayer (CESL) 232 may be formed on the SiO_(X) layer 1502, on thesource/drain features 1302 and 1404, and along the top and sides of thegate stacks 1002. The CESL 232 may be substantially similar to thatdescribed above and may be deposited by any suitable technique includingALD, CVD, High-Density Plasma CVD (HDP-CVD), and/or other suitabletechniques and may be formed to any suitable thickness. In someexamples, the CESL 232 has a thickness between about 1 nm and about 50nm.

Referring to block 920 of FIG. 9B and referring still to FIG. 16, afirst Inter-Level Dielectric (ILD) layer 1602 is formed on the workpiece1000. The first ILD layer 1602 may be substantially similar to thatdescribed above and may be formed by any suitable process including CVD,PVD, spin-on deposition, and/or other suitable processes.

Referring to block 922 of FIG. 9B and to FIG. 17, a chemical mechanicalpolish/planarization (CMP) process is performed on the workpiece 1000 toremove the first ILD layer 1602 and/or the CESL 232 from the top of theplaceholder gate stacks 1002. The CMP process may be followed by an etchback process to remove any remaining ILD layer 1602 material or CESL 232material from the placeholder gate stacks 1002.

Referring to block 924 of FIG. 9B and to FIG. 18, the placeholder gatestacks 1002 are removed and replaced with functional gate stacks 1802.The materials of the placeholder gate stacks 1002 may be removed by oneor more etching processes (e.g., wet etching, dry etching, RIE) usingetchants configured to selectively etch the materials of the placeholdergate stacks 1002 without significant etching of the surroundingmaterials, such as the fins 208, the sidewall spacers 230, the CESL 232,the first ILD layer 1602, etc.

The functional gate stacks 1802 are then formed in recesses defined byremoving the placeholder gate stacks 1002. The functional gate stacks1802 may be substantially similar to those described above and mayinclude layers such as an interfacial layer 218, a gate dielectric 220,a capping layer 222, work function layer(s) 224, electrode fill 226,and/or a gate cap 228, each substantially as described above.

Referring to block 926 of FIG. 9B and to FIG. 19, a second ILD layer1902 may be formed on the existing ILD layer 1602 and on the functionalgate stacks 1802. This may be performed substantially as described inblock 920, and the second ILD layer 1902 may be substantially similar tothe first ILD layer 1602.

Referring to block 928 of FIG. 9B and to FIG. 20, trenches 2002 areformed in the ILD layers 1602 and 1902 for source/drain contacts. Thetrenches 2002 expose the source/drain features 1302 and 1404 atlocations where conductive features of the interconnect are to beformed. This may be performed substantially as described in block 104 ofFIG. 1. In some examples, a patterned photoresist is formed on theworkpiece 1000 that selectively exposes portions the ILD layers 1602 and1902 to etch. The exposed portions of the ILD layers 1602 and 1902 arethen etched as described above.

The trenches 2002 may expose may expose the SiO_(X) layers 1502 of thesource/drain features 1302 and 1404 in whole or in part. Referring toblock 930 of FIG. 9B and to FIG. 21, the exposed portions of the SiO_(X)layers 1502 are removed in a precleaning process that is configured toavoid significant removal of the Ge-rich layer 1504. This may beperformed substantially as described in block 108 of FIG. 1, and mayinclude a dry cleaning process, a wet cleaning process, RIE, and/orother suitable cleaning methods. The precleaning process may leave someof the sloping side portions of the SiO_(X) layers 1502 and some of thehorizontal central portion of the SiO_(X) layers 1502 depending on thewidth of the trench and the subsequent contact to be formed.

Referring to block 932 of FIG. 9B and to FIG. 22, the processes ofblocks 110-114 of FIG. 1 are performed on the workpiece 1000. This mayinclude performing a silicide/germanide forming process as described inblock 110 to form a silicide layer 2202 on the silicon-containingsource/drain features 1302 in the nFET region 204, and a germanide layer2204 and a nitridized germanide cap 2206 on the SiGE-containingsource/drain features 1404 in the pFET region 206. In various examples,the silicide layer 2202 has a thickness between about 1 nm and about 1nn, the remaining Ge-rich layer 1504 has a thickness of less than about2 nm, the germanide layer 2204 has a thickness between about 2 nm andabout 5 nm, and the nitridized germanide cap 2206 has a thicknessbetween about 1 nm and about 3 nm. In various examples, the nitridizedgermanide cap 2206 has a nitrogen concentration between about 15 andabout 40 atomic percent.

Source/drain contacts 702 may be formed in the trenches 2002 that extendthrough the remaining SiO_(X) layers 1502 to couple to the source/drainfeatures 1302 and 1404 as described in block 112. In particular, thecontacts 702 may physically and electrically couple to the silicidelayer 2202 of the nFET source/drain features 1302 and to the germanidelayer 2204 and/or nitridized germanide cap 2206 of the pFET source/drainfeatures 1404. The contacts 702 may be substantially as described aboveand may include one or more layers of conductive materials such asmetals, metal oxides, metal nitrides, and/or combinations thereof.

A planarization process may be performed to remove portions of thecontact material that are above the ILD layers 1602 and 1902 asdescribed in block 114.

Referring to block 934 of FIG. 9B, the workpiece 1000 is provided forfurther fabrication. In various examples, this includes forming aremainder of an electrical interconnect structure, dicing, packaging,and other fabrication processes.

Thus, the present disclosure provides examples of an integrated circuitwith an improved interface between a source/drain feature and asource/drain contact and examples of a method for forming the integratedcircuit. In some embodiments, the method includes receiving a substratehaving a source/drain feature disposed on the substrate. Thesource/drain feature includes a first semiconductor element and a secondsemiconductor element. The first semiconductor element of thesource/drain feature is oxidized to produce an oxide of the firstsemiconductor element on the source/drain feature and a region of thesource/drain feature with a greater concentration of the secondsemiconductor element than a remainder of the source/drain feature. Theoxide of the first semiconductor element is removed, and a contact isformed that is electrically coupled to the source/drain feature. In somesuch embodiments, the first semiconductor element includes silicon andthe second semiconductor element includes germanium. In some suchembodiments, metal is introduced into the region of the source/drainfeature to form a germanide layer of the source/drain feature. In somesuch embodiments, nitrogen is introduced into the germanide layer toform a nitridized cap layer on a remainder of the germanide layer, andthe contact physically couples to the nitridized cap layer. In some suchembodiments, the region of the source/drain feature is substantiallyfree of the first semiconductor element. In some such embodiments, thesource/drain feature is a pFET source/drain feature, and the substratefurther has an nFET source/drain feature disposed on the substrate. ThenFET source/drain feature includes the first semiconductor element. Theoxidizing of the first semiconductor element of the pFET source/drainfeature further forms the oxide of the first semiconductor element onthe nFET source/drain feature, and the removing of the oxide removes theoxide from the pFET source/drain feature and the nFET source/drainfeature. In some such embodiments, the nFET source/drain feature issubstantially free of the second semiconductor element. In some suchembodiments, the substrate further includes an inter-level dielectricdisposed on the source/drain feature, and a recess is formed in theinter-level dielectric that exposes the source/drain feature. Theoxidizing and the removing of the oxide are performed through therecess. In some such embodiments, the contact is formed in the recess.

In further embodiments, a substrate is received having an nFET regionwith an nFET source/drain feature and a pFET region with a pFETsource/drain feature. The pFET source/drain feature includes a firstsemiconductor material and a second semiconductor material. An oxidationprocess is performed on the nFET source/drain feature and the pFETsource/drain feature to form an oxide layer on the nFET source/drainfeature and on the pFET source/drain feature. The oxidation processfurther forms a region of the pFET source/drain feature with a greaterconcentration of the second semiconductor material than a remainder ofthe pFET source/drain feature. The oxide layer is removed from the nFETsource/drain feature and from the pFET source/drain feature. A firstcontact is formed electrically coupled to the nFET source/drain featureand a second contact is formed electrically coupled to the pFETsource/drain feature. In some such embodiments, the first semiconductormaterial includes silicon and the second semiconductor material includesgermanium. In some such embodiments, the nFET source/drain feature issubstantially free of germanium. In some such embodiments, a germanidelayer is formed from the region of the pFET source/drain feature withthe greater concentration of the second semiconductor material. In somesuch embodiments, the region of the pFET source/drain feature issubstantially free of the first semiconductor material. In some suchembodiments, the substrate further includes an inter-level dielectricdisposed on the nFET source/drain feature and on the pFET source/drainfeature. A first recess is formed in the inter-level dielectric thatexposes the nFET source/drain feature, and a second recess is formed inthe inter-level dielectric that exposes the pFET source/drain feature.The performing of the oxidation process and the removing of the oxidelayer are performed through the first recess and the second recess.

In yet further embodiments, a substrate is received having a findisposed on the substrate. A SiGe source/drain feature is formed on thefin. A top surface of the SiGe source/drain feature is oxidized to forman oxide layer on the SiGe source/drain feature and a region of the SiGesource/drain feature with a greater concentration of Ge than a remainderof the SiGe source/drain feature. The oxide layer is removed from theSiGe source/drain feature, and a contact is formed that couples to theSiGe source/drain feature. In some such embodiments, the region of theSiGe source/drain feature is substantially free of silicon. In some suchembodiments, a metal is introduced into the region of the SiGesource/drain feature to form a germanide layer. In some suchembodiments, nitrogen is introduced into the germanide layer to form anitridized cap layer on the germanide layer. In some such embodiments,an inter-level dielectric is formed on the inter-level dielectric afterthe oxidizing of the top surface and prior to the removing of the oxidelayer.

In further embodiments, a device includes a substrate having a finextending from a remainder of the substrate, a source/drain featuredisposed on the fin, and a contact coupling to the source/drain feature.The source/drain feature includes a SiGe semiconductor, and a topportion of the source/drain feature has a different germaniumconcentration than a bottom portion of the source/drain feature. In somesuch embodiments, the device also includes a dielectric layer thatincludes silicon oxide disposed on the top portion of the source/drainfeature. The contact extends through the dielectric layer. In some suchembodiments, the device also includes an etch stop layer disposed on thedielectric layer, and the contact extends through the etch stop layer.In some such embodiments, the dielectric layer includes a side portionthat slopes upward and a horizontal central portion extending from theside portion that physically contacts the contact.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: receiving a substrate havinga first source/drain feature and a second source/drain feature disposedthereupon, wherein the first source/drain feature includes a firstsemiconductor element and a second semiconductor element and the secondsource/drain feature includes the first semiconductor element, whereinthe first source/drain feature is part of a first transistor having afirst conductivity type and the second source/drain feature is part of asecond transistor having a second conductivity type that is opposite thefirst conductivity type; oxidizing the first semiconductor element ofthe first source/drain feature to produce an oxide layer that includesthe first semiconductor element on the first source/drain feature and aregion of the first source/drain feature with a greater concentration ofthe second semiconductor element than a remainder of the firstsource/drain feature, wherein the oxidizing of the first semiconductorelement of the first source/drain feature further includes oxidizing ofthe first semiconductor element of the second source/drain feature toform the oxide layer on the second source/drain feature; removing theoxide layer from the first source/drain feature and the secondsource/drain feature; and forming a contact electrically coupled to thefirst source/drain feature.
 2. The method of claim 1, wherein the firstsemiconductor element includes silicon and the second semiconductorelement includes germanium.
 3. The method of claim 2 further comprisingintroducing metal to the region of the first source/drain feature toform a germanide layer of the first source/drain feature.
 4. The methodof claim 3 further comprising introducing nitrogen to the germanidelayer to form a nitridized cap layer on a remainder of the germanidelayer, wherein the contact physically couples to the nitridized caplayer.
 5. The method of claim 1, wherein the region of the firstsource/drain feature is substantially free of the first semiconductorelement.
 6. The method of claim 1, wherein the second source/drainfeature is substantially free of the second semiconductor element. 7.The method of claim 1, wherein: the substrate further includes aninter-level dielectric disposed on the first source/drain feature; themethod further comprises forming a recess in the inter-level dielectricthat exposes the first source/drain feature; and the oxidizing and theremoving of the oxide layer are performed through the recess.
 8. Themethod of claim 7, wherein the contact is formed in the recess.
 9. Themethod of claim 1, wherein the first conductivity type is p-type and thesecond conductivity type is n-type.
 10. A method comprising: receiving asubstrate having an nFET region with an nFET source/drain feature and apFET region with a pFET source/drain feature, wherein the pFETsource/drain feature includes a first semiconductor material and asecond semiconductor material; performing an oxidation process on thenFET source/drain feature and the pFET source/drain feature to form anoxide layer on the nFET source/drain feature and on the pFETsource/drain feature, wherein the oxidation process further forms aregion of the pFET source/drain feature with a greater concentration ofthe second semiconductor material than a remainder of the pFETsource/drain feature; removing the oxide layer from the nFETsource/drain feature and from the pFET source/drain feature; and forminga first contact electrically coupled to the nFET source/drain featureand a second contact electrically coupled to the pFET source/drainfeature.
 11. The method of claim 10, wherein the first semiconductormaterial includes silicon and the second semiconductor material includesgermanium.
 12. The method of claim 11, wherein the nFET source/drainfeature is substantially free of germanium.
 13. The method of claim 11further comprising forming a germanide layer from the region of the pFETsource/drain feature with the greater concentration of the secondsemiconductor material.
 14. The method of claim 10, wherein the regionof the pFET source/drain feature is substantially free of the firstsemiconductor material.
 15. The method of claim 10, wherein: thesubstrate further includes an inter-level dielectric disposed on thenFET source/drain feature and on the pFET source/drain feature; themethod further comprises forming a first recess in the inter-leveldielectric that exposes the nFET source/drain feature and a secondrecess in the inter-level dielectric that exposes the pFET source/drainfeature; and the performing of the oxidation process and the removing ofthe oxide layer are performed through the first recess and the secondrecess.
 16. A method comprising: receiving a substrate having asource/drain feature disposed thereupon, wherein the source/drainfeature includes a first semiconductor element; oxidizing the firstsemiconductor element of the source/drain feature to produce an oxidelayer that includes the first semiconductor element on the source/drainfeature; removing a first portion of the oxide layer to expose a portionof the source/drain feature, wherein a second portion of the oxide layerremains disposed on the source/drain feature after the removing of thefirst portion of the oxide layer; and forming a contact electricallycoupled to the source/drain feature, wherein the second portion of theoxide layer remains disposed on the source/drain feature after theforming of the contact.
 17. The method of claim 16, wherein thesource/drain feature further includes a second semiconductor elementthat is different from the first semiconductor element, wherein theoxidizing of the first semiconductor element of the source/drain featureto produce the oxide layer further generates a region of thesource/drain feature with a greater concentration of the secondsemiconductor element than a remainder of the source/drain feature. 18.The method of claim 17, further comprising: introducing a metal materialto the region of the source/drain feature to form a germanide layer ofthe source/drain feature; and introducing nitrogen to the germanidelayer to form a nitridized cap layer on a remainder of the germanidelayer, wherein the contact physically contacts the nitridized cap layer.19. The method of claim 16, further comprising forming a silicidefeature on the exposed portion of the source/drain feature, and whereinthe silicide feature interfaces with the second portion of the oxidelayer.
 20. The method of claim 16, further comprising: forming a firstgate stack on the substrate, wherein the source/drain feature isassociated with the first grate stack; forming a contact etch stop layerdirectly on the oxide layer; replacing a first gate electrode of thefirst gate stack with a second gate electrode formed of a differentmaterial, wherein the replacing of the first gate electrode of the firstgate stack with the second gate electrode occurs prior to the removingof the first portion of the oxide layer to expose the portion of thesource/drain feature; and removing a portion of the contact etch stoplayer to expose the first portion of the oxide layer.